What causes my Mac G5 to turn on by itself and run the fans at highest speed indefinitely?

Answer 1

Your system may have crashed at some point, but I doubt that will cause you to not be able to use it at all. Truthfully, when the fans run up in RPM, one of two things are happening, the processor is possibly 'stuck' and at that moment the fan was being accellerated, or it's getting too hot. Make sure the computer has space around it and isn't completely covered. It should have space to move air around it. Also, if the area you are keeping it in has a heat source nearby, that would be a problem. Macs (expecially the higher speed newer ones) create a lot of heat all by themselves. They need air circulation, or the processor will eventually have a 'melt-down.'

I suppose if you have the CPU near a heat source, the computer COULD turn itself on so it could run the fans to cool, but the fact is that if this is occurring at a regular interval, then someone has set the auto schedule where it will start-up or shut-down automatically. You should also check the system preferences (energy saver) to see if the "restart after power failure" is checked. A fluctuation in the power would then cause the computer to start randomly as well.